A conventional system used for this purpose comprises a transmitter/receiver assembly or duplexer connected, on the one hand, to telephone equipment such as a subscriber's instrument by means of a coupler and a telephone line and, on the other hand, to a similar assembly by means of a transmission link terminating in analog/digital and digital/analog converters.
In the transmitter section of such a duplexer the voice signal to be transmitted is sampled and reproduced in the form of amplitude-modulated pulses. These modulated pulses are then tansmitted to an analog/digital converter which encodes each of them into groups of n bits. These groups of bits are fed, by the transmission link, to a digital/analog converter in which the encoded pulses are decoded and transformed into pulses whose amplitude is a function of the value of the encloded pulses received. The pulses leaving this digital/analog converter are transmitted to the receiver section of another duplexing device where they are demodulated and filtered to reconstitute the emitted signal.
Thus, each of these devices includes a modulation channel with an outgoing amplifier and a demodulation channel with an incoming amplifier connected to a telephone instrument by a coupler.
The modulation channel comprises a low-frequency fixed-gain amplifier, a low-pass filter arranged to suppress frequencies higher than 4000 Hz, and a modulator, all connected in series. The amplified and filtered signal is transmitted to the modulator which is controlled by a sampling system common to a number of such devices. The output from the modulator is connected to the analog/digital converter either directly or through an omnibus multiplex link or bus bar common to this plurality of devices.
The demodulation channel receives the amplitude-modulated pulses either directly or through an omnibus multiplex link common to a number of devices associated with the digital/analog converter. This channel comprises a demodulator, an amplifier and a low-pass filter connected in series.
The demodulator selects the amplitude-modulated pulses and charges a capacitor during the time the demodulator is sampling. The capacitor integrates the pulses, which are transmitted to the amplifier and then to the filter which suppresses all frequencies higher than 4000 Hz.
The coupler, which is adjusted by a balancer, joins the two-wire circuit of the telephone instrument to the four-wire circuit necessary in order that each channel may have two wires and may be isolated from the other channel. It also serves as a switch from the transmitted and received signals. A signal transmitted by the telephone instrument is switched by the coupler into the modulation channel, which processes it with the object of transmitting it to the analog/digital converter. Conversely, a series of pulses supplied by the digital/analog converter and received by the demodulation channel is, after processing by the latter, transmitted to the coupler which switches it to the telephone instrument.
However, this known duplexer suffers from serious drawbacks. Firstly, the coupler is constituted by a differential transformer with multiple windings requiring a balancer. As a result, the device is expensive and heavy. Furthermore, owing to the need for a transformer, the device cannot be produced in the form of an integrated semiconductor circuit.